An Equalizer is used for to compensate the amplitude, or phase, of an electric signal over frequency or time. The compensation may be necessary because of long lines, or other filters, that disturbs the signal. For example, equalizers are widely used in cable TV systems where long cables attenuate signals more at higher frequencies. It was also soon found out that the capacitances between long telephone lines attenuated higher frequencies of the speech. The solution, an equalizer, was the Pupin box. See Wikipedia for Pupin!